Device for flat illumination of an object field

ABSTRACT

The invention is directed to a device for flat illumination of an object field in an optical instrument and to an optical instrument with a device of this kind. Optical instruments of this type are, for example, microscopes, including microlithography simulation microscopes in which a flat illumination, i.e., illumination extending beyond a singular object point, of the object to be examined is required. The device comprises a laser light source and a light-conducting cable with at least one optical fiber through which the light from the laser light source is guided to the object field. The optical fiber is constructed and dimensioned in such a way that the intensity of the illumination light within the cross section of the optical fiber becomes increasingly more uniform along the path from the input-side end to the output-side end, and the illumination light is directed from the output-side end of the optical fiber to the object with substantially homogeneous intensity distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of PCT application No. PCT/EP02/07039,filed Jun. 26, 2002 and German application No. 101 30 821.3, filed Jun.26, 2001, the complete disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a device for flat illumination of an objectfield in an optical instrument and to an optical instrument with adevice of this kind. Optical instruments of this type are, for example,microscopes, including microlithography simulation microscopes in whicha flat illumination, i.e., illumination extending beyond a singularobject point, of the object to be examined is required.

b) Description of the Related Art

In instruments of the type mentioned above, the homogeneity of theillumination of the object field is particularly important as regardsthe quality of the recorded images of the object field. To this extent,the problem with using laser light sources for generating theillumination light is that the intensity of the light is unevenlydistributed within the beam cross section, which has a disadvantageouseffect on image quality.

Other variations in the brightness of the illumination whichdisadvantageously affect the image quality and, therefore, the resultsof evaluation are caused by low-frequency amplitude modulations orcoherent noise (speckle) in coherent light sources. Therefore, animportant concern consists in reducing these interfering influences asfar as possible.

For this purpose, in a solution which is already known, a rotatingdiffusion screen or scatter disk is arranged in the beam path betweenthe laser light source and the object field in order to reduce theintensity modulation of the laser light.

Further, it is known to use light-conducting cable in connection withoptical instruments. These light-conducting cables have been usedheretofore to transmit light from the source to the object in the mostflexible manner possible. Usually, the light-conducting cable is used totransfer illumination light within the spectral range from longwave(λ>400 nm) to infrared.

OBJECT AND SUMMARY OF THE INVENTION

On this basis, it is the primary object of the invention to provideanother possibility for homogenizing the illumination of an extendedobject field.

For this purpose, a device of the type mentioned in the beginningcomprises a laser light source for generating illumination light and alight-conducting cable with at least one optical fiber through which theillumination light reaches the object. The optical fiber is constructedand dimensioned in such a way that the intensity distribution of theillumination light within its cross section becomes increasingly moreuniform along the path from the input-side end to the output-side endand the illumination light is directed from the output-side end of theoptical fiber to the object with substantially homogeneous intensitydistribution.

The effect achieved in this way is similar to that of a rotating scatterdisk. In contrast to the rotating scatter disk, however, the inventivesolution is a static system offering particularly extensive protectionagainst interference due to the absence of moving components. Inaddition, the drive device and associated controls required for arotating scatter disk are dispensed with.

The light-conducting cable can be arranged in a flexible manner with nospecial restrictions with respect to its position, so that greatercompactness can also be achieved in instrument construction in contrastto a scatter disk.

A reduction in the intensity modulation over the beam cross section canbe realized by means of different configuration parameters of thelight-conducting cable. This can be accomplished, for example, bypredetermining the length of the optical fiber and/or the diameter ofthe optical fiber and/or the material used for the optical fiber, sothat the spatial coherence of the laser light is destroyed. By reductionof the intensity modulation is meant herein a reduction of the intensityvariations over the cross section at the output of the light-conductingcable to less than 10%, preferably to less than 1%.

In a preferred construction, the laser light source generates light inthe UV range, preferably in a wavelength of 386 nm, 365 nm, 266 nm, 257nm, 248 nm, 213 nm, 211 nm or 193 nm at a bandwidth of about ±2 nm. Theoptical fiber used is preferably a multimode light guide. In this case,the diameter of the light-conducting part has a multiple of thewavelength of the light coming from the source.

It has proven advantageous when the length of the optical fiber is atleast 30 cm. With excessive length, the intensity losses are too high sothat the maximum length should currently be limited for technicalreasons preferably to about 100 cm. Insofar as optical fibers withgreater stability or fewer losses are available, their lengths can alsobe greater.

In another advantageous construction of the invention, the light sourceis a pulsed laser light source. In comparison to scatter disks, it is tobe observed when using optical fibers according to the invention thathomogenization is successfully achieved already with small pulsenumbers, so that an illumination apparatus of this kind is very wellsuited to optimization of measured throughput in measuring processes.

To further improve and homogenize the illumination, one or more rotatingscatter disks can also be arranged in front of and/or behind thelight-conducting cable.

When two scatter disks are provided, it is advantageous as regardshomogenization when they rotate in opposite directions. In order toachieve the scattering effect, the scatter disks are granular, forexample. When using a pulsed laser light source, the granulation, i.e.,the size of the individual grains, is adapted to the pulse number andpulse duration and to the rotating speed of the scatter disk. It wouldalso be conceivable to adapt to the characteristic coherence length. Thepurpose of this adaptation is so that the light can pass essentiallythrough the same point on the scatter disk during one pulse, e.g.,during 10 ns, i.e., the scatter disk is quasi-stationary with respect toits optical effect, whereas rotation continues in a noticeable mannerduring the time segment between every two pulses. Accordingly, thegranulation of the scatter disk as well as the non-homogeneity ofintensity over the beam cross section of the light source are averaged.A holographic profile can also be used instead of granulation.

The invention further proposes an optical instrument, particularly amicrolithography simulation microscope, which comprises an observationdevice which can be oriented to an object field and an illuminationdevice of the type mentioned above.

The illumination device is preferably arranged next to the observationdevice as a structural unit. The illumination device is connected to theobservation device by a device for conducting light. This can be carriedout, for example, by deflecting mirrors or another light-conductingcable.

Of course, it is also possible to couple light directly into theobservation device via the light-conducting cable of the illuminationdevice.

The coupling of light into the observation device can be carried outusing the principle of incident light or transmitted light.

In another advantageous construction of the invention, the observationdevice is configured so as to scan a plurality of different objectpoints in the object field.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the invention will be described more fully withreference to an embodiment example shown in a drawing, FIG. 1, whichshows a microscope with an illumination device according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The microscope 1, which is constructed in this case as amicrolithography simulation microscope by way of example, is outfittedwith an observation device 2 and an illumination device 3. In this way,for example, masks for semiconductor or wafer fabrication as well asother objects can be examined.

The observation device 2 comprises an objective 4 which is directedtoward a movable table 5 with a receptacle for an object O to beexamined. Further, illumination optics 6 are provided for uniformillumination of a flat object field on the object O. In the presentembodiment example, illumination is provided based on the principle oftransmitted light. However, it is also possible to illuminate andexamine the object O based on incident light.

The observation device 2 shown here is constructed for scanning aplurality of object points in the object field. Image informationobtained in this way is recorded in a video camera 7 connected to theobservation device 2. However, other recording media outfitted with animage-recording sensor could also be used instead of the video camera 7.Beyond this, the observation device 2 can make possible visualobservation of the flat-illuminated object field when constructed in asuitable manner.

The illumination device 3 mentioned above which is shown here as aseparate structural unit is associated with the microscope 1. Theillumination device 3 comprises a laser light source 8 for generatingthe light by which the object field is to be illuminated. The laserlight source 8 generates coherent light in the UV range and isconstructed in this case, by way of example, as a pulsed laser lightsource.

Before the light generated by the laser light source 8 strikes an objectO, it is conducted through a light-conducting cable 9. Thelight-conducting cable 9 comprises one or more optical fibers and servesto interrupt the coherence of the light emitted by the laser lightsource 8. There are different possibilities for accomplishing this. Forexample, a homogenization of the intensity distribution over the crosssection of the optical fiber depends on length, diameter and material.Through a specific selection of these configuration parameters, anappreciable homogenization of the intensity distribution is caused overthe cross section between the input (the end on the input side) and theoutput (the emission-side end) of the light-conducting cable.

A light-conducting cable 9 with an individual optical fiber constructedas a multimode light guide is used in the embodiment example shown. Thelength of the optical fiber is 30 to 100 cm, preferably 50 to 60 cm. Thelight-conducting cable 9 can be arranged in the illumination device 3 inany manner desired. Also, fiber portions extending to and fro can bebundled so as to form a kind of multiwire cable.

Further, the illumination device 3 comprises an optical in-couplingdevice 10 in the form of a lens or lens arrangement, possibly also witha diaphragm, by which the light of the laser light source 8 is coupledinto the light-conducting cable 9. Further, an optical out-couplingdevice 11 is provided. This again comprises one or more lenses andpossibly additional diaphragms.

Another light-conducting cable 12 serves to conduct the light emitted bythe illumination device 3 to the object O via the optical out-couplingdevice 11. Suitable optical couplers 13 and 14 are provided at the endsof the other light-conducting cable 12 for connecting to theillumination optics 6 and to the illumination device 3.

A highly homogeneous illumination of an object field on the object O isachieved already by the configuration described above. In particular,the occurrence of coherent noise is sharply reduced through thelight-conducting cable 9, so that interfering influences due to thecoherent noise or speckle are reduced or even prevented in the image ofthe object field.

Instead of the other light-conducting cable 12 mentioned above, thelight can also be transmitted by deflecting mirrors. Moreover, it ispossible to connect the illumination device 3 directly to theillumination optics 6.

Further, in the present embodiment example a rotating scatter disk 15 isprovided in the illumination device 3. In this case, the rotatingscatter disk 15 is positioned between the optical in-coupling device 10and the light-conducting cable 9. The associated drive device by whichthe scatter disk 15 can be set in rotation is not shown.

The rotational speed of the scatter disk is selected in such a way thatthis scatter disk is quasi-stationary for the duration of a laser pulse,e.g., 10 ns, but is moved further in the interval between two laserpulses before the next laser pulse is emitted (repetition frequency is,e.g., 200 Hz). Successive pulses accordingly pass through differentportions of the scatter disk 15.

Speeds in the range of several centimeters per second are sufficient forthe repetition frequency mentioned above. The grain size of thegranulation is in the range of 0.1 mm.

A second scatter disk (not shown) can be provided in addition toreinforce the homogenizing effect caused by the scatter disk 15. Thissecond scatter disk is stationary or rotates in the opposite directiondepending on the construction of the invention. When the second scatterdisk is stationary, it should advantageously be arranged in front of therotating scatter disk considered in the radiating direction.

The arrangement of the scatter disks in relation to the light-conductingcable 9, the optical in-coupling device 10 and the optical out-couplingdevice 11 can also be relatively freely selected, i.e., the scatter disk15 can be arranged in front of or behind these components. This alsoapplies to any additional scatter disks which may possibly be providedin the illumination device 3. Scatter disks provided with holographicpatterns which are configured with respect to a homogenizing effect overthe beam cross section can also be used instead of granular scatterdisks.

In another modification of the embodiment example, it is also possibleto integrate the optical out-coupling device 11 in the illuminationoptics 6, so that the light-conducting cable 9 takes over the functionof the other light-conducting cable 12.

The illumination device 3 described in connection with the microscope 1can also be used for other purposes, specifically, for example, anywherethat the most homogeneous possible flat illumination of an extendedobject field or measurement field with an optical instrument is desired.An example for this would be a device for photolithographic exposure ofsemiconductor substrates or wafers.

While the foregoing description of the drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made therein without departing from the true spirit andscope of the present invention.

Reference Numbers:

-   -   1 microscope    -   2 observation device    -   3 illumination device    -   4 objective    -   5 table    -   6 illumination optics    -   7 video camera    -   8 laser light source    -   9 light-conducting cable    -   10 in-coupling device    -   11 out-coupling device    -   12 light-conducting cable    -   13, 14 couplers    -   15 scatter disk    -   O object

1. A device for flat illumination of an object field in an opticalinstrument and to an optical instrument for observation of objects,particularly a microscope, comprising: a laser light source forgenerating illumination light; and a light-conducting cable with atleast one optical fiber through which the illumination light is guidedto the object; said optical fiber being constructed and dimensioned insuch a way that the intensity distribution of the illumination lightbeing directed from the output-side end of the optical fiber to theobject with substantially homogeneous intensity distribution; wherein atlease one rotating scatter disk is arranged in front of and/or behindthe light-conducting cable and two scatter disks with opposite rotatingdirections are provided.
 2. A device for flat illumination of an objectfield in an optical instrument and to an optical instrument forobservation of objects, particularly a microscope, comprising: a laserlight source for generating illumination light; and a light-conductingcable with at least one optical fiber through which the illuminationlight is guided to the object; said optical fiber being constructed anddimensioned in such a way that the intensity distribution of theillumination light being directed from the output-side end of theoptical fiber to the object with substantially homogeneous intensitydistribution; wherein at lease one rotating scatter disk is arranged infront of and/or behind the light-conducting cable and two scatter disksare provided, one of which rotates while the second is stationary, andthe stationary scatter disk is arranged in front of the rotating scatterdisk considered in the radiating direction.
 3. A device for flatillumination of an object field in an optical instrument and to anoptical instrument for observation of objects, particularly amicroscope, comprising: a laser light source for generating illuminationlight; and a light-conducting cable with at least one optical fiberthrough which the illumination light is guided to the object; saidoptical fiber being constructed and dimensioned in such a way that theintensity distribution of the illumination light being directed from theoutput-side end of the optical fiber to the object with substantiallyhomogeneous intensity distribution; wherein at lease one rotatingscatter disk is arranged in front of and/or behind the light-conductingcable and two scatter disks with opposite rotating directions areprovided and the scatter disks are provided with a granular orholographically generated, optically active structure.